The present invention relates to an optical wavelength control apparatus for controlling the optical wavelength of a light-emitting element, e.g., a laser diode, used in optical fiber communication.
For multi-channel wavelength multiplex transmission in the optical communication system, it is very important to control the oscillation center wavelength of the individual laser diode.
According to a method of controlling the oscillation center wavelength of a laser diode, the temperature of the diode element is changed. The temperature of a Peltier effect element increases or decreases in accordance with the direction and magnitude of a current flowing through the element. Accordingly, wavelength control of the laser diode can be performed by using this effect.
This method is currently often utilized in wavelength control of a laser diode. As an example, a laser wavelength control apparatus shown in Japanese Patent Laid-Open No. 8-37334 will be described with reference to FIG. 3.
A beam emitted by a laser diode 42 is input to an optical high-pass filter 46 and optical low-pass filter 51, both having a narrow band. The tuning wavelengths of the two optical filters 46 and 51 are set to sandwich a target optical center wavelength .lambda.. Beams that have passed through the optical high-pass filter 46 and optical low-pass filter 51 are respectively converted into electrical signals by photodiodes (PDs) 48 and 53.
If the optical center wavelength is shifted to the long wavelength side due to some reason, the potential of the photodiode 48 connected to the optical high-pass filter 46 increases, and the potential of the photodiode 53 connected to the optical low-pass filter 51 decreases. These changes are detected by a comparator 56 to operate a temperature adjustment circuit 58. The temperature adjustment circuit 58 performs a control operation to increase (or decrease) the temperature of a Peltier effect element 41, thereby restoring the optical wavelength to the short wavelength range.
When the optical wavelength is shifted to the short wavelength side, the potential of the photodiode 48 connected to the optical high-pass filter 46 decreases, and the potential of the photodiode 53 connected to the optical low-pass filter 51 increases. These changes are detected by the comparator 56 to operate the temperature adjustment circuit 58. The temperature adjustment circuit 58 performs a control operation to decrease (or increase) the temperature of the Peltier effect element 41, thereby restoring the optical wavelength to the long wavelength side. Wavelength control of the laser diode 42 can be performed in this manner.
In the laser diode optical wavelength control apparatus described above, if the optical center wavelength is largely shifted to fall outside the range of the optical low-pass filter 51 or optical high-pass filter 46 for some reason (e.g., a power supply is turned on), outputs from the photodiodes 48 and 53 disappear and the output from the comparator 56 becomes unstable. Therefore, the optical wavelength of the laser diode 42 becomes uncontrollable.
In the wavelength control apparatus for the laser diode 42 described above, once the optical wavelength becomes uncontrollable, it cannot be restored to the controllable state.